var mergeTwoLists = function (list1, list2) {
  let l1 = list1
  let l2 = list2
  let res = new ListNode()
  let head = res

  while (l1 && l2) {
    if (l1.val <= l2.val) {
      res.next = l1
      l1 = l1.next
    } else {
      res.next = l2
      l2 = l2.next
    }
    res = res.next
  }

  res.next = li !== null ? l1 : l2

  return head.next
};